His current deal ran out in the summer, but he decided his future was in Cumbria - and his current club decided to sell now to ensure they received a transfer fee.
"We offered Jeff a new improved contract for next season but he declined it and accepted a better offer from Carlisle," said a Vale spokesman.
"From a footballing point of view, we had to make that decision whether to sell him now or let him go for free at the end of the season.
"Hopefully now we can now go on and strengthen the side for next season."